Advances continue to be made in the manufacture of solid-state electronic devices, resulting in increasing functionality, density and performance of the integrated circuits. The amount of power dissipated, and accordingly the amount of heat generated, by modern integrated circuits generally increases with increases in the density and speed of the circuits. Removal of heat produced by the integrated circuits therefore continues to be of significant concern of modern integrated circuit package and system designers, considering the loss of performance and the degradation in reliability of integrated circuits when operated at elevated temperatures.
In addition, the trend toward more compact electronic systems is also continuing, exacerbating the thermal problem produced by the high-complexity and high-performance integrated circuits. For example, laptop or notebook sized computers have recently become quite popular, with continuing market pressure toward even smaller computer systems such as personal digital assistants (PDAs). However, these small systems eliminate many of the techniques for heat removal available for large-scale computer systems, such as the use of fans for convection cooling of the integrated circuits. As such, many modern systems utilize thermal conduction as the primary mode of heat removal from the integrated circuits in the system.
By way of further background, integrated circuit packages of the ball-grid-array (BGA) type have recently become popular in the field. Conventional BGA packages are similar in layout and arrangement to the pin-grid-array (PGA) packages, in providing a rectangular or square array of connections on the underside of the integrated circuit package. Instead of the pin connectors used in PGA packages, however, BGA packages utilize a solder ball located at each connector location. As is known in the art, the BGA package is attached to a printed circuit board by reflowing the solder balls to make connection to conductors at the surface of the printed circuit board. The BGA package provides the important advantage of being self-aligning, as the surface tension of the solder will tend to pull the BGA package into proper alignment with the corresponding conductors on the printed circuit board.
By way of further background, BGA integrated circuit packages of various types are known, including those of both the "cavity-up" and the "cavity-down" type. Conventional cavity-up BGA packages mount the integrated circuit chip face-up into a cavity (or onto the surface) of the package substrate, attach wire bonds between the package and the chip on this top side, and then either transfer mold or otherwise dispense a plastic over the chip and bond wires to provide environmental protection to the chip and wires. The solder balls are provided on the side of the substrate opposite the chip, and the packaged chip is then mounted to the system printed circuit board. It is known to use plated vias through the substrate at locations underlying the chip, but that are offset from conductors on the printed circuit board to which the packaged chip is mounted, to provide thermal conduction from the chip to its external environment.
Conventional cavity-down integrated circuit packages mount the integrated circuit chip into the cavity of, or onto the surface of, the substrate, followed by attachment of bond wires thereto and molding of the plastic around the chip. In this type of package, the solder balls are provided on the same side of the substrate as the chip, so that the chip is disposed upside-down after attachment of the packaged chip to the circuit board. It is known to mount the chip to a thermally conductive slug disposed within the substrate of a cavity-down package, with the slug exposed on its opposite side from the chip after mounting. A heat sink may be mounted to the exposed upper surface of the slug, so that the heat sink extends upwardly and facilitates convection cooling. This arrangement is not particularly well suited for modern small computing systems, such as laptop, notebook-sized, or hand-held computers, as these systems are not able to house a fan, and indeed are designed to have no convection flow of air within the housing ("zero-air-flow").
It is therefore an object of the present invention to provide an integrated circuit package having improved thermal conduction from the packaged chip to the circuit board upon which it is mounted.
It is a further object of the present invention to provide such a package which is arranged in the cavity-up orientation.
It is a further object of the present invention to provide such a package that is of the ball-grid-array (BGA) type.
It is a further object of the present invention to provide such a package that reduces the need for convection cooling.
It is a further object of the present invention to provide such a package which provides solder connections of larger cross-sectional area for improved thermal conductivity.
It is a further object of the present invention to provide such a package which provides such large area thermal solder connections using the same size solder ball as that used for electrical connections.
Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification together with its drawings.